1. Field of the Invention
The invention relates to the construction industry and, in particular, concerns a method of providing lateral strengthening of wall structures using factory manufactured, field installed A-frame shear assembly with ductile attachment members.
2. Description of the Related Art
Low-rise, commercial, institutional and residential (single and multi-family) buildings comprise the majority of buildings in the United States. Within this group of buildings, by far the most prevalent type of structure is the light framed structure, specifically wood or cold-formed/light-gauge steel framing. In the typical light framed building structure, as in any other building structure, the basic structural design goals is to ensure the safe performance of the building under anticipated loading conditions. Safe performance may include, but is not limited to, one or more of the following performance objectives: operational/immediate occupancy performance, life safety performance and collapse prevention performance (FEMA-273 “NEHRP Guidelines for the Seismic Rehabilitation of Buildings,”1997).
The loads to be considered in design vary in the degree by which they can be reasonably (in a probabilistic sense) defined. Fundamentally though, there are two types of load to consider in design: gravity and lateral loads. Gravity loads, as the name implies, act vertically and they have one characteristic that makes them more deterministic than lateral loads—they can be controlled to some extent. Lateral loads (for example those induced by earthquakes and hurricane/tornado winds) are unpredictable in both occurrence and magnitude. In design for lateral load, the conventional philosophy has been to provide a lateral load resisting structural system that is strong enough to resist the maximum expected design event. In earthquake resistant design, this philosophy is further augmented by the additional requirement for inelastic deformation capability (ductility) of the lateral load resisting system. Inherent in this ductility requirement is the understanding that under the maximum design event, a building will undergo some amount of damage associated with the design performance objectives stated above.
In conventional light framed building construction, gravity and lateral load resistance is achieved essentially by a stick frame (studs, joists, rafter and trusses) for the gravity loads and sheathing attached to the stick frame for lateral loads. Loads are typically generated at different levels within the building and must be carried to the foundation via the combined action of the stick frame and the attached sheathing. This combined action implies that some elements of the gravity and lateral load systems will be common. As a result, failure of any one of these common elements under one loading condition (say lateral) can compromise the integrity of the entire system under the other condition.
Sheathed stick-framed walls that are designed to resist lateral loads are commonly referred to in the literature as shear walls or vertical diaphragms. The details of how a shear wall resists lateral load are quite complex. Generally though, the basic mechanism of resistance is achieved by a transfer of load from the point where they are generated into the frame, from the frame into the sheathing, from the sheathing back into the frame and from the frame into the foundation. Because of this load path, each component in the load path needs to have capacity of transferring the full load for a shear wall to work as expected. In other words, the performance of the shear wall is controlled by its weakest link. In earthquake resistant design, performance is attained by having the capacity to transfer loads at the foundation be higher than the capacity of the sheathing to frame attachment.
The sheathing materials commonly used in light frame shear wall construction typically include plywood, oriented strand board, fiberboard, gypsum wallboard/sheathing board, siding and sheet steel. The sheathing is typically attached to the frame with nails, staples or screws. In some cases, as may be the case with light gauge steel framing, sheet steel may be attached to the frame by clinching, welding or an adhesive. Additionally, in cold-formed steel construction lateral resistance may also be accomplished with flat-strap x-bracing. These generic systems, which are typically included in building codes, are not the only means of providing lateral resistance. In fact, other prefabricated systems are available for use as braced wall components. The primary benefits of these systems are improved performance due to the quality control associated with fabrication of the component and ease of installation in the field.
The aforementioned prefabricated systems, though more advanced than shear and x-braced walls, provide a response similar to that of the conventional field-built shear wall. That is, to develop a certain level of lateral resistance under the design event, these systems must undergo significant inelastic deformation (damage) which in turn results in damage to the contents and other non-structural components of the building. Furthermore, conventional shear walls and other prefabricated panel systems used in light framed buildings, may have to be comparatively large or strong to withstand the magnitude of lateral loads and/or deformations that are generated in design events or as limited by building codes. For example, most building codes limit the inelastic story drift or lateral displacement to between 2 inches and 2.5 inches for an 8-foot wall height in all types of buildings. To meet this limitation, the braced wall (shear wall, x-bracing or prefabricated system) must generally be ductile (ability to deform), strong and stiff. As the stiffness and strength of bracing components increase, the demands placed on other components of the structure also increases, thereby requiring larger members. It can be appreciated that multi-story buildings will be more susceptible to larger lateral forces/deformations often necessitating even larger lateral bracing structures. Increased spatial requirements for the lateral bracing system exacerbates the problem of a limited amount of space in walls of smaller lengths.
Hence, there is a need for a lateral bracing system that is easy to install, is comparatively small in size so that it can be readily installed in walls having short lengths, has the ability to dissipate energy without significant damage to the structures (and its components), has the ability to reduce the magnitude of deformations and forces induced in the building, improves life-safety of occupants and protects building functionality. To this end, there is a need for a prefabricated internal shear assembly with a mechanical lateral motion dampening device.